1. Field of the Invention
The present invention relates to a communication apparatus and particularly to a communication apparatus suitable for use in a multiplex transmission system in which a plurality of control circuits (hereinafter referred to as ECU) are capable of transmitting and receiving communication signals through a common transmission line.
2. Background Information
A communication signal to be transmitted among ECUs is expressed by a combination of square wave signals. Information is interpreted by comparing a signal received by an ECU with a predetermined threshold value and providing demodulation.
However, such a threshold value is affected by GND voltage differences among ECUs, as well as fluctuations in respective power supply voltages. As a result errors are generated during modulation and demodulation operations. Hence, accurate transmission of communication signals is significantly degraded.
Moreover, because square wave signals rise and fall sharply, a communication signal with combinations of various square wave signals will include a considerable degree of harmonics.
When baseband transmitting through a common transmission line, because the communication signal is a square wave signal, baseband harmonics are radiated and superposed over radio frequency bands, resulting in adverse interference effects on close by devices, such as interference on the reception of radio programs. Consequently, it is essential to take adequate and effective measures against harmonics radiation noise.
As a counter measure, communication signals are not transmitted in square wave form. Instead, the signals are being modulated to a trapezoidal wave shape. The trapezoidal wave signals received by each ECU, are demodulated to the original square wave form, on the basis of a constant threshold value preset in the receiving circuit.
In the above example, spectrum power of harmonic signals is reduced because of trapezoidal wave signals. As a result, the properties of noise radiation is reduced to a very small amount.
However, when compared to a square wave based transmission signal, trapezoidal wave signals have a problem in that errors are generated during modulation and demodulation such as can result from fluctuation in threshold voltage or in communication signal voltage.
In addition, although trapezoidal wave based transmission signals are effective in reducing noise radiation at low transmission rates such as at about 10 kbps at intermediate transmission rates, such as 100 kbps, where a baseband of 50 Khz is common, such signals create problems. When a baseband frequency is very close to an AM radio frequency band (about 500 to 700 Khz) harmonics radiation noise generates adverse effects.